Semiconductor integrated circuits contain large numbers of electronic devices such as diodes and transistors built onto a single crystal or chip, often made of silicon. Since these devices are so small, their operational integrity is very susceptible to imperfections or impurities in the crystal. The failure of a single transistor in a circuit may render that circuit non-functional.
In order to circumvent this problem, the semiconductor industry regularly builds redundant circuits on the same chip. Therefore, if a faulty circuit is discovered during testing, the faulty circuit can be disabled and its redundant circuit enabled. Often, this switching to a redundant circuit is accomplished by blowing certain fuses built into the circuitry of the chip. Those fuses signal the location of the defective element and enable a substitute element in a redundant circuit bank.
In the case of memory ICs, memory cells are usually arranged in rows and columns. Each memory cell is addressed by a particular row and column. By blowing the correct combination of fuses, circuitry which addresses the faulty elements, can be isolated and replaced with circuitry which addressed a corresponding redundant element.
A common method of selectively blowing fuses in a fuse bank is by the use of a laser, wherein energy from the laser is selectively directed toward the various fuses. The laser melts the selected fuses and isolates the defective circuits. This process is commonly known as laser break-link programming or laser programming.
The process of laser break-link programming, however, does have its drawbacks. Primary among them is the required size of the fuse bank in order to effectively use a laser. As a laser is directed against surface of a chip, it generally burns a round crater onto whatever surface the laser strikes. When used to melt a fuse, the diameter of the crater made by the laser must be wider than the fuse to ensure that the fuse is completely severed. However, the crater caused by the laser can not be too wide or else the laser will melt fuses positioned next to the targeted fuse. Consequently, prior art fuse banks need to be manufactured with relatively large spaces between the various fuses in the fuse bank. Due to semiconductor circuit manufacturing tolerances and laser light tolerances, the typical fuse bank must have a spacing of at least 4.5 .mu.M to 5.4 .mu.M in order for laser break-link programming to be used. This required spacing often causes multiple fuse banks to be present at various places on a integrated circuit chip. This consumes a significant amount of space on the chip, limiting further miniaturizing of the integrated circuits.
The prior art is replete with different fuse bank structures that attempt to reduce the size of fuse banks. Such prior art references are exemplified by U.S. Pat. No. 5,185,291 to Fisher, entitled METHOD OF MAKING SEVERABLE CONDUCTIVE PATH IN AN INTEGRATED-CIRCUIT DEVICE; U.S. Pat. No. 4,910,418 to Graham et al. entitled SEMICONDUCTOR FUSE PROGRAMMABLE ARRAY STRUCTURE; U.S. Pat. No. 4,935,801 to McClure et al. entitled METALLIC FUSE WITH OPTICALLY ABSORPTIVE LAYER; and U.S. Pat. No. 5,025,300 to Billing et al, entitled INTEGRATED CIRCUITS HAVING IMPROVED FUSIBLE LINKS. Such prior art references provide structures that absorb laser radiation and increase the effectiveness of the laser. However, such prior art systems are still limited by the size of the laser, wherein the spacing between fusible links must be large enough to accommodate the diameter of the laser beam.
It is therefor an objective of the present invention to provide a fuse bank with an increased density of fusible links, wherein the distance between separate elements in the fuse bank is less than the diameter of the laser beam used to sever those elements.
It is a further objective of the present invention to increase the density of a fuse bank by 100% while increasing the size of the fuse bank by less than 50%.